The tunnel will overtake Japan's 53.9 km Seikan rail tunnel as the longest in the world and push the 50.5 km Channel Tunnel linking Britain and France into third place.

The project, which cost more than $12 billion to build, was endorsed by Swiss voters in a referendum in 1992. Voters then backed a proposal from environmental groups to move all freight travelling through Switzerland from road to rail two years later.

The completed tunnel travels up to 2.5 km below the surface of the mountains above and through rock.

Engineers had to dig and blast through 73 different kinds of rock, some as hard as granite and others as soft as sugar.

Nine workers were killed during the work.

When full services begin in December, the journey time for travellers between Zurich and Milan will be reduced by an hour to two hours and 40 minutes.

About 260 freight trains and 65 passenger trains will pass through the tunnel in as little as 17 minutes.